The shield or the serum separately is an Easter egg. The cameo of Samuel L. Jackson as Nick Fury at the end of Iron Man, by itself, is fan service. And the appearance of Robert Downey Jr. as Tony Stark in The Incredible Hulk is — well, I don’t know yet.

But taken together, these represent an effort by Marvel, or at least the writers and directors of these two films, to subtly and quickly create a “universe” — something not easily done in movies designed to stand on their own.